postprocess: add --no-fail-fast, progress numbers, comment diffs, exclude file of all errors, etc.

[kkysen]

This fixes/cleans up and adds a few things to c2rust-postprocess:

• Collects all the prompts first (doing all of the filtering first) and then runs them so that we can list progress. If we want to parallelize LLM calls later, it's also set up to do that more easily now.
• Adds --fail-fast/--no-fail-fast. With --no-fail-fast, errors are collected and then shown all at the end instead of ending as soon as the first error is reached. This is useful when trying to determine with functions to exclude.
• Prints diffs of the C vs. Rust comments when they don't match so we can see what's going wrong.
• Prints an exclude file for all of the comment mismatch failures.
• Uses logging.info for printing the highlighted Rust so that we can suppress it with --log-level like the other similar things we print.